Metal-insulator transition and localization in quasicrystall alloys

Metal-insulator transition was observed in icosahedral quasicrystals (Formula presented)(Formula presented)(Formula presented)(Formula presented) as x changed from 5 to 0. It was found that conductivity data of the insulating samples (x⩽3.5) could be analyzed in terms of variable range hopping, i.e., σ=(Formula presented) exp[-((Formula presented)/T(Formula presented)], with p=1/2 for x=2, 2.5, 3, and 3.5 and p=1/4 for x=0, over the temperature range from 0.45 to 10 K. The localization length was found to be of the order of intercluster distance in the quasicrystalline structure. At decreasing x, magnetoresistance (MR) was found to change smoothly from weak-localization behavior in the metallic regime to anomalous features in the insulating regime which are distinctly different from those seen in other well-studied insulators. The persistence of positive MR in insulating (Formula presented)(Formula presented)(Formula presented)(Formula presented) alloys suggests that the backscattering effect plays an important role for localized states in quasicrystalline materials. Specific heat measurement on the insulating x=2 sample yielded γ=0.38 mJ/g atom (Formula presented), and carrier density was found to be ∼(Formula presented) at 4.2 K from Hall effect measurement, which are at least as large as those of metallic Al-Cu-Fe quasicrystals. The finding of spatially localized states in quasiperiodic alloys cannot be explained satisfactorily by existing theories. © 1996 The American Physical Society.